1. Field of the Invention
The present invention relates to a silicon carbide semiconductor device.
2. Description of Related Art
A silicon carbide semiconductor device is a device that is formed by using silicon carbide (SiC) as semiconductor material. JP-A-H10-308510, corresponding to U.S. Pat. No. 6,573,534, discloses an SiC semiconductor device including a planer metal-oxide-semiconductor field-effect transistor (MOSFET). JP-A-H5-55594 discloses a configuration for improving a surge resistance of a silicon semiconductor device. According to the configuration, a p type well region that functions as a deep base region is located below a p type base region.
The inventors have revealed the following idea, as a related art. A surge resistance of the planer MOSFET disclosed in JP-A-H10-308510 can be improved when a p type well region is formed below a p type base region in a manner similar to that disclosed in JP-A-H5-55594.
Discussion is given below on the above idea, as a related art. In manufacturing an SiC semiconductor device, since impurities are difficult to diffuse in SiC, it is difficult to employ a diffusion process that is widely used in manufacturing a silicon device. It is hence difficult to form a p type well layer to a deep position in SiC by the diffusion process. Thus, a breakdown can easily take place not in a cell portion but a periphery of the cell portion. Since an area of the periphery of the cell portion is relatively small, a surge energy is difficult to be absorbed. Additionally, even when a breakdown takes place in the cell portion, a breakdown occurs at a corner part of a p type base region. In such a case, an electric potential below an n+ type source region easily increases. A parasitic transistor provided by the n+ type source region, a p type base region and an n type drift region becomes an on state, causing element destruction. Alternatively, the p type base region may be biased, and a substrate bias effect may non-uniformly form a channel and cause element destruction.
The above difficulties associated with the surge resistance are relevant to not only a planer MOSFET but also a vertical type semiconductor device, in which a current flows between a front surface and a rear surface of the substrate. Such a vertical type semiconductor device is, for example, a trench gate MOSFET, an insulated gate bipolar transistor (IGBT), a Schottky barrier diode (SBD), or a junction barrier diode (JBS).